1. Field of the Invention
The present invention relates to a method for forming a ball on a bonding wire used in wire bonding and more particularly to a method that applies a high voltage across the tip end of the wire and a discharge electrode so as to generate an electric discharge between them to form the ball on the bonding wire.
2. Prior Art
Discharge electrode systems employed in wire bonding include a fixed type discharge electrode system, in which the discharge electrode is positionally fixed, and a movable type discharge electrode system, in which the discharge electrode is moved by a solenoid, etc. The fixed type discharge electrode system is disclosed in, for example, Japanese Patent Application Kokai No. 7-263480; and the movable type discharge electrode system is disclosed in, for instance, Japanese Patent Application Laid-Open (Kokai) Nos. H5-36748, H5-102233 and H7-176560.
In the fixed type discharge electrode systems, a discharge electrode is positioned on one side of the tip end of the wire so that the electrode can be located as close as possible to the tip end of the wire with a space in between which can avoid contact with the wire when the capillary (which the wire passes through) is lowered. In the movable type discharge electrode system, on the other hand, a discharge electrode is moved so as to be positioned directly beneath the tip end of the wire, and then an electric discharge is generated by the discharge electrode to form a ball on the wire. Thus in this system, the discharge electrode is retracted from beneath the capillary after the ball is formed.
In the fixed type discharge electrode systems, a driving means or control means which moves the discharge electrode is not required unlike the movable type discharge electrode system. As a result, the fixed type discharge electrode systems have a number of superior features. The overall weight of the bonding head can be reduced, high-speed operation is possible, the cost of the apparatus can be low and maintenance is easy.
In the movable type discharge electrode systems, the electric discharge is generated with the discharge electrode moved directly beneath the tip end of the wire. Accordingly, this system can form a ball so as not to be eccentric with respect to the center of the wire. On the other hand, though the fixed type discharge electrode systems have various advantages as described above, the electric discharge is performed with the discharge electrode positioned on one side of the tip end of the wire; accordingly, it is likely that eccentric balls or warped balls are formed. For this reason, the movable type discharge electrode systems are generally more widely used than the fixed type discharge electrode systems.
A ball formation by the fixed type discharge electrode system is shown in FIGS. 3 and 4.
As seen from FIG. 3, in the fixed type discharge electrode system, a discharge electrode 3 is positioned to one side of the tip end of a wire 2 which passes through a capillary 1. Accordingly, when the electric discharge is generated, discharge sparks 4a, 4b and 4c are, as shown in FIG. 4(a), discharged toward the wire 2 from the side of the wire 2; and therefore, the tip end of the wire 2 melts from the directions of the discharge sparks 4a, 4b and 4c. As a result, the formed balls 5a, 5b and 5c are eccentric or get warped as shown by the ball 5d in FIG. 4(b) which deviates from the center of the diameter of the wire 2.
In addition, if any contaminant adheres to the tip end of the discharge electrode 3, the electric discharge generates from the side surface (upper surface in FIG. 4(a)) of the discharge electrode 3 as shown by the discharge spark 4c. In such a case, since the wire 2 begins to melt not from the tip end but from the position of the ball 5c, the wire 2 is cut at an intermediate point before a ball is formed, or an extremely small ball is formed on the wire 2. Even if the wire 2 is not cut at an intermediate point, a ball with a distorted shape such as that shown in FIG. 4(c) at 5e tends so be formed. This problems leads to an inconsistent ball diameter.